Detection and classification of gamma ray emitters has attained heightened importance in the protection of vulnerable targets and populaces from high energy explosives. Many nuclear explosives emit gamma rays, due to radioactive decay of the materials comprising the explosives. However, many less harmful and non-explosive materials also emit gamma rays. Therefore, it is desirable to be able to identify, and whenever possible, distinguish between the types of gamma ray emitters in an unknown material, possibly further concealed inside of a container or vehicle of some type, such as a car, van, cargo container, etc.
Many types of materials emit gamma rays that appear very close together on a gamma spectrograph. Scintillator detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The intensity of the light emitted can be used to determine the type of material that is emitting the gamma rays. Scintillator detectors may also be used to detect other types of radiation, such as alpha, beta, and x-rays. High energy resolution scintillator detectors are useful for resolving closely spaced gamma ray lines in order to distinguish between gamma emitters producing closely spaced gamma ray lines.
Detection sensitivity for weak gamma ray sources and rapid unambiguous isotope identification is principally dependent on energy resolution, and is also enhanced by a high effective atomic number of the detector material. Generally, gamma ray detectors are characterized by their energy resolution. Resolution can be stated in absolute or relative terms. For consistency, all resolution terms are stated in relative terms herein. A common way of expressing detector resolution is with Full Width at Half Maximum (FWHM) divided by the peak energy. This equates to the width of the gamma ray peak on a spectral graph at half of the highest point on the peak distribution.
NaI(Tl) is known in the art as an excellent scintillation counter, and to be particularly useful as a gamma ray spectrometer. The combination of the high atomic number, a density of 3.67 g/cm3, a high light yield (38,000 photons/MeV), energy resolution of about 7% at 662 keV, and lack of intrinsic radioactivity make NaI(Tl) one of the most important scintillators. Moreover, another advantage associate with NaI scintillators is the ease of its production in single crystal form. However, while NaI has the rock salt (NaCl) cubic structure that yields isotropic mechanical and thermal properties, which are highly desirable during the crystal growth process, it is also quite hygroscopic.
Improved radioisotope identification detectors based on gamma spectroscopy that can rapidly detect and identify weak sources require high sensitivity detector materials offering better energy resolution, and high effective atomic number. The scintillators currently providing the highest energy resolution of as favorable as 2.6% at 662 keV and sizes larger than 1″ dia.×1″ height are LaBr3(Ce) and SrI2(Eu). These crystals have non-cubic structures and are hygroscopic, both of these factors reducing the growth yield of crystals, and limiting the largest available scintillator size to approximately 2″ dia.×3″ height. In the case of strontium iodide the purification and removal of water in the dopant, EuI2 adds to processing time. LaBr3(Ce) also has intrinsic radioactivity that, for large volume detectors and low count rate applications, adds unwanted background and impedes performance for radioisotope identification of weak sources.
Another class of crystals being explored for application in gamma ray spectroscopy is the family of rare-earth elpasolites. These are quaternary compounds where most crystallize in the double perovskite structure with Cs2LiYCl6 (CLYC) and others having been reported. Their general formula is AB2MX6 where X− is a halide ion (F, Cl, Br, or I). The main attractive features of this structure are the incorporation of6Li on the A site for neutron detection and Ce3+ activator on the B site, however the light yield of 20,000 photons/MeV for CLYC is somewhat modest among gamma ray scintillators and the presence of both Li and rare earth ions cause the crystal to be hygroscopic.